This invention relates to the problem of adjusting, accurately and promptly, the position of mirrors in an interferometer. Proper working of the interferometer requires alignment of the system by extremely fine adjustment of mirror position in at least one arm of the interferometer, assuming use of an interferometer in which the source-supplied radiation is partially reflected and partially transmitted by a beamsplitter, thereby creating two arms, one of which has a fixed length, and the other of which has a variable length for scanning purposes.
Alignment is accomplished with the aid of any convenient monochromatic (laser) beam. The laser beam is directed into the interferometer where it passes through the beamsplitter and, therefore, has a component in the fixed-length arm and a component in the variable-length arm. The exiting laser beams returning from the respective arms should coincide, if the system is properly aligned. If they do not coincide, they will show spaced dots on an intercepting surface.
The process of mirror adjustment requires extensive manipulation of the mirror surface until the two exiting laser dots impinge on the same point on a temporarily erected viewing screen. The purpose of initial adjustment, or alignment, of the interferometer is to obtain coincidence of the two dots. After the initial adjustment to bring the dots into coincidence, a much more difficult procedure is required to adjust the interference pattern of an expanded beam. It is the usual practice to accomplish alignment by adjusting the position of the mirror, or mirrors, in the fixed-length arm of the interferometer.
Various means are used for such mirror adjustments. One of the preferred means is a combination of rotatable wedges behind the mirror, which cause minute diametrical tilting movements of the mirror. Two wedges are used, each of which is rotatable around a common axis, which also constitutes the center of the mirror, and one of which is secured to the mirror. Rotating the two wedges in the same direction causes a certain curvilinear motion of the laser dot which is being aligned. Rotating the two wedges in opposite directions, causes a different curvilinear motion of the same laser dot. Alternate manipulation of the wedges in the same and in opposite directions gradually reaches the desired alignment, with the two laser dots merged into one. A functional advantage of the wedge alignment structure, as distinguished from other adjusting means, is its relative stability, because the surface areas of the wedges are relatively large, and are as near as possible in size to the surface area of the adjustable mirror.
For many years, various problems have been encountered during the wedge mirror adjustment process. The wedges, and the supporting plate which one of them engages, are metallic. Friction between the metallic surfaces tends to prevent a smooth motion during adjustment, unless lubricant is used. However, the use of fluid lubricant, which is the general practice, is the cause of additional problems. When grease is used between the surfaces which have relative movement during adjustment, pressure forcing the surfaces together causes "squeezing out" of some of the grease. Any localized "opening up" between engaging surfaces tends to "draw in" grease. Either of these changes in the grease thickness disturbs the alignment. It is, therefore, necessary to align the mirror, wait until the effects of displacing the grease have been experienced, and then realign the mirror; and this sequence may have to be repeated several times. This may become a tedious and time-consuming process. In some instances, several hours may be required for realignment, which even then may not be fully stabilized.
One of the expedients which has been tried, for the purpose of minimizing the problems caused by the lubricant, is illustrated in FIG. 3 of the drawings. The bearing surfaces between the relatively movable members have been reduced to an annular area extending around the peripheral portion of the mirror. While this use of annular engaging surfaces, by reducing the area of engagement, and thereby reducing the amount of lubricant, diminishes the lubricant-caused problems, it has the disadvantage of also diminishing stability because the full available contact area is not being used.
The mirror adjustment problems are troublesome with relatively long wavelengths, as in the medium and far infrared spectra. When the interferometer is used in short wavelength regions, alignment using known methods becomes extremely difficult. This difficulty has become particularly serious in the MIDAC spectrometer system referred to as the FTPL system, which uses photo-luminescence as the source of radiation in the spectrometer. This system is disclosed in Auth application Ser. No. 641,835, filed Aug. 17, 1984, as a continuation-in-part of application Ser. No. 555,607, filed Nov. 28, 1983. The inventor and assignee of the present application are the same as in the cited FTPL applications.
The shorter wavelength systems will not tolerate alignment imperfections which might be acceptable with longer wavelengths.